1. Field of the Invention
The present invention relates to an improvement of the signal-to-noise ratio of a reproduction signal from an optical disc apparatus.
2. Background Art
With the commercialization of Blu-ray Disc (BD) that employs blue semiconductor laser and a high-NA objective lens, the optical disc has virtually reached a limit in terms of optical system resolution, and for further increase in capacity, increasing the number of layers is believed to offer a major solution. In a multilayer optical disc, given the need to have substantially equal amounts of light detected from each layer, the reflectivity of certain layers inevitably needs to be reduced. However, in addition to the increases in the capacity of optical discs, their transfer speeds are also becoming faster due to the need to increase the speed at which videos, for example, are copied. Thus, it is becoming increasingly difficult to ensure a sufficient S/N ratio in a reproduction signal as is. Therefore, in order to increase the number of layers and achieve faster speeds in the future, it is indispensable to increase the S/N of the detection signal.
Technologies for increasing the S/N of a reproduction signal from optical discs are disclosed in Patent Documents 1 and 2, for example. In these technologies, in order to achieve higher S/N ratios in the reproduction signal from a magneto-optical disc, the light from a semiconductor laser is branched before the optical disc is irradiated therewith. The light that is not shone on the optical disc is composed with the reflected light from the optical disc such that they interfere with each other, so that the amplitude of a weak signal can be amplified by increasing the amount of the light that is not shone on the optical disc. In differential detection between the transmitted light that passed through a polarizing beam splitter and the reflected light, which is a conventionally employed technique for signal detection in magneto-optical discs, essentially the original incident polarization component is caused to interfere with a polarization component produced by rotation of polarization by the magneto-optical disc that is perpendicular to the incident polarization direction, so as to amplify the perpendicular polarization component using the incident polarized light for detection purposes. Thus, while the signal can be increased by increasing the original incident polarization component, the intensity of the light incident on the optical disc needs to be controlled to be lower than a certain level so as not to erase or overwrite data. To address such need, in the aforementioned conventional technologies, the light that is to be caused to interference with the signal light is separated in advance and then caused to interference with the signal light without focusing it onto the disc, where the intensity of the interfering light for signal amplification purposes can be increased regardless of the optical intensity on the disc surface. In this way, it becomes possible, in principle, to increase the S/N ratio by increasing the intensity within an allowed range thereof, relative to the noise in the amplifier for converting an optical current from the photodetector into voltage or the shot noise caused in the photodetector, for example.
In Patent Document 1, two beams of light are caused to interference with each other and the interference intensity is detected. In this case, the optical path length of the interfering, non-disc-reflected light is made variable so as to ensure an interference signal amplitude. In Patent Document 2, in addition to the interference intensity detection, differential detection is also carried out so as to cancel the intensity component of each light that does not contribute to the relevant signal, thus cancelling the noise components of such light and achieving a higher S/N ratio. In this case, the differential detection involves a beam splitter having no polarization.
Patent Document 1: JP Patent Publication (Kokai) No. 5-342678 A (1993)
Patent Document 2: JP Patent Publication (Kokai) No. 6-223433 A (1994)